This invention relates to an earth reentry thermal protection system material for a specific temperature application and, more particularly, to a smooth coated waterproof thermally stable flexible insulating material, considerably lower in cost and more easily applied compared to materials used in higher temperature zones and which would have been used had not this invention occurred.
Space vehicles are subjected to increases in temperature during ascent and reentry. To protect the metallic structure of the vehicle during such time it is customary to provide a heat shield. During the earlier space programs in which the manned vehicles were used for single trips, the heat shields were formed of ablative material. However, with the advent of the Space Shuttle which uses a vehicle designed to make numerous trips into space, it was necessary to develop a reuseable thermal protection system. The Space Shuttle Orbiter is in some respects similar to a large airplane and consequently various areas of the vehicle are subject to varying surface temperature extremes. For example, the nose and leading edges of the wings are subject to the greatest heating and require a high temperature carbon composite material with internal insulation which provides protection for surface temperatures in excess of 2300.degree. F. (1533.degree. K.). The undersides of the wings and fuselages require a material that will provide protection for surface temperatures up to 2300.degree. F. (1533.degree. K.). Certain portions of the upper side of the fuselage only require protection for surface temperatures up to 1200.degree. F. (922.degree. K.). There are many other portions which require protection to 700.degree. F. (644.degree. K.). It has been determined that highly refined chemically pure silica fibers regidized with a high purity silica binder would be used for all surface areas subjected to 2300.degree. F. (1533.degree. K.), or less. This material is cut into thin, square shaped tiles. The tiles, known as L1-900, are manufactured by Lockheed Missiles & Space Co., Inc. and have relatively low strength, extremely high temperature resistance, and extremely low coefficient of thermal expansion as compared to metals. These tile insulators have a density of 9 pounds/cu.ft. (0.144 Grams/cu. cm.) and generally are effective to repetitive cycles of temperatures to 2300.degree. F. (1533.degree. K.). Because of their relatively low strength, however, the tiles cannot be used for load-bearing applications and must be secured to the protected structure by a means which will minimize transfer of strains from the metal structure to the tile.
An early method for attaching these tiles is disclosed in U.S. Pat. No. 3,920,339 entitled "Strain Arrestor Plate" wherein a strain arrestor plate formed of a material having a similar coefficient of expansion to that of the tile is interposed between the insulating tile and the structure. While this construction solved the problem of thermal expansion, the problem of stress compatability during cold orbital conditions remained. To solve both problems, a strain isolator, made of needled Nomex felt replaced the use of the strain arrestor plate and U.S. Pat. Application Ser. No. 786,913, now U.S. 4,124,733, entitled "Thermal Insulation Attaching Means", which is a continuation-in-part of Ser. No. 555,750, now abandoned, covers such development.
As mentioned before, the rigidized silica tiles were proposed for all areas under 2300.degree. F. (1533.degree. K.) with the thickness of the tiles being determined by the temperature requirements. However, it was found to avoid excessive breakage that the rigized silica tiles could be made no thinner than 0.2 inches (0.508 cm). In many areas the 0.2 inch (0.508 cm) minimum thickness over-protected the areas and created excess weight. Moreover, it was found to be impractical, because of physical characteristics, to make tiles larger than 8 inches.times. 8 inches (20.32 cm.times.20.32 cm) and thus, each of these fragile tiles had to be individually attached to the structure, therefore, installation was most time-consuming and resulted in high breakage rates.
In determining characteristics of the strain isolator noted above, it was discovered that the material had sufficient thermal capacity to insulate against temperatures up to 900.degree. F. (756.degree. K.) but it was not waterproof and had a tendency to shrink after going through temperature cycling. The present invention involves a new and unique use of Nomex felt to overcome the disadvantages of the prior art tile. With the present invention, improved methods and apparatus are herewith provided for obtaining larger sized insulation material with more flexibility and less weight yet provide sufficient temperature isolation for temperatures expected to be encountered by the Space Shuttle orbiter in specific zones.